[LLVMdev] Suggestion: Support union types in IR

[Talin]

I've been thinking about how to represent unions or "disjoint types" in LLVM
IR. At the moment, the only way I know to achieve this right now is to
create a struct that is as large as the largest type in the union and then
bitcast it to access the fields contained within. However, that requires
that the frontend know the sizes of all of the various low-level types (the
"size_t" problem, which has been discussed before), otherwise you get
problems trying to mix pointer and non-pointer types.
It seems to me that adding a union type to the IR would be a logical
extension to the language. The syntax for declaring a union would be similar
to that of declaring a struct. To access a union member, you would use
GetElementPointer, just as if it were a struct. The only difference is that
in this case, the GEP doesn't actually modify the address, it merely returns
the input argument as a different type. In all other ways, unions would be
treated like structs, except that the size of the union would always be the
size of the largest member, and all of the fields within the union would be
located located at relative offset zero.

One issue would be how to represent the union in text form. One alternative
is to use the same syntax for structs, except replace the comma with a
vertical bar character, as in this example:

    {int|float}

The drawback to this approach is that you can't tell whether it is a struct
or union until after you have parsed the first argument. If that turns out
to be too inconvenient for the parser, then some unique two-character
sequence will have to be used, such as:

   {|int|float|}

Another idea is to use struct syntax with a keyword prefix:

   union {int, float}

The specific details of syntax I will leave to others.

Unions could of course be combined with other types:

   {{int|float}, bool} *
   n = getelementptr i32 0, i32 0, i32 1

So in the above example, the GEP returns a pointer to the float field.

-- 
-- Talin
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